The present invention relates to a flow meter for detecting a flow rate of fluid.
Particularly, the present invention is suitable for a flow meter in which a member or element forming a branch passage is supported within a main passage of fluid to measure a flow rate of fluid flowing through the branch passage.
Conventionally, as a fluid flow meter of the kind referred to above, an air flow meter has been known which detects a quantity of suction or intake air drawn into an engine of an automobile.
For example, "GAS FLOW RATE MEASURING DEVICE" disclosed in Japanese Patent Unexamined Publication No. 62-235525 has been known. This measuring device is provided with a shell-like element supported substantially at a central portion within a resin housing forming therein an intake passage for axial movement, to measure a quantity of air by a quantity of movement of the element.
However, since the flow meter has a moving portion, there are problems that the flow meter is weak in oscillation or vibration, a change in characteristic occurs due to friction, and the like.
Further, as a flow meter provided with no moving portion, a thermal flow meter has been known in which an exothermic body or a heating element is arranged within a fluid flow.
For example, in Japanese Patent Unexamined Publication No. 60-185118, a structure has been disclosed in which a cylindrical element is supported substantially at a center of an intake passage, and a branch passage is formed within the element so that a sensor is arranged within the branch passages.
In a technique disclosed in Japanese Unexamined Publication No. 60-185118, however, since an outlet of the branch passage the downstream side of stays which support a branch passage element within the main passage, there is a fear that turbulent flow within the main passage generated by the stays acts on an outlet opening. If the turbulent flow in the main passage acts on the output outlet opening, turbulence occurs in flow within the branch passage, and fluctuation is given to a flow rate introduced into the branch passage. Particularly, fluctuation occurs in the ratio between the flow rate in the main passage and the flow rate within the branch passage. Thus, there is a problem that it is impossible to accurately measure the flow rate within the intake passage.